Heat-expandable microspheres, which comprise a shell of a thermoplastic resin and a blowing agent encapsulated therein, are generally called heat-expandable microcapsules. The thermoplastic resin usually includes vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic acid ester copolymers. The blowing agent mainly employed includes hydrocarbons, such as isobutane and isopentane. (Refer to PTL 1).
The process for producing such heat-expandable microspheres includes a well-known process in which an oily mixture containing a monomer component having one radically-polymerizable double bond, a blowing agent such as aliphatic hydrocarbons, a cross-linking agent having at least two radically-polymerizable double bonds and a polymerization initiator is dispersed in an aqueous dispersion medium containing, for example, a dispersion stabilizer such as colloidal silica or magnesium hydroxide, and the dispersion is subjected to suspension polymerization (refer to PTL 2).
Heat-expandable microspheres have been employed in various applications including imparting design or functionality to articles or reducing the weight of articles, and demand for higher performance of such articles in each application is increasing recently. Along with such a trend, demand for higher performance of heat-expandable microspheres is also increasing.
An example of such applications is a process of manufacturing formed products or sheets by expanding heat-expandable microspheres in vinyl chloride paste or compositions of resins such as polyurethane resin during the processing of such compositions.
The expansion performance of heat-expandable microspheres in a composition being subjected to processing is greatly influenced by the processing conditions, and such influence sometimes causes defects in the resultant formed products or sheets.
Specifically, the influence causes problems such as shrinkage of heat-expandable microspheres due to the escape of vaporized blowing agent from the microspheres heated at high temperature or for a long time, even if the heat-expandable microspheres show sufficient thermal expansion performance in heating at a comparatively low temperature and for a short time. Such deteriorated thermal expansion performance of the microspheres leads to the problems including a poor effect of imparting design or functionality to formed products or sheets or failure to reduce their weight.
For solving the significant temperature-dependency of heat-expandable microspheres in processing, a process for producing a polymer for heat-expandable microspheres that is less temperature dependent has been proposed. The process produces the polymer by increasing the amount of a cross-linking agent (polymerizable monomer having at least two polymerizable double bonds) added to an oily mixture containing polymerizable monomers in order to increase the degree of cross-linking of the polymer (as a whole) which forms the shell of heat-expandable microspheres so as to make the polymer less temperature dependent. The process, however, caused a problem in that the increased cross-linking density of the polymer can lead to decreased thermoplasticity of the whole of the shell. Such a shell does not sufficiently soften along with the increased internal pressure of the encapsulated blowing agent, and thus the resultant heat-expandable microspheres have poor thermal expansion performance.